There are known arrangements for tuning a laser to operate at selected frequencies throughout a wide range of frequencies. Frequency-tunable semiconductor lasers are attractive devices for optical frequency division multiplex transmission systems. In a tunable arrangement, a distributed Bragg reflection laser provides a large tuning range when different values of injection or bias current are injected into the phase control and the distributed Bragg reflector regions of the laser.
A known laser control arrangement is disclosed, for example, in U.S. Pat. No. 4,914,666, which is hereby incorporated by reference. This arrangement includes a laser tunable in response to different values of a control signal. A digital processor determines control signal values which produce different operating frequencies. The exact operating frequencies are defined by the resonances of a Fabry-Perot resonator. The control signal values are stored in the digital processor for ready retrieval. During operation, one of the control signal values, representing a desired operating frequency, is retrieved from storage and is applied to the single frequency laser to lock the laser at one of the resonances of the Fabry-Perot resonator. If the laser characteristic curve (i.e., laser frequency as a function of bias current) has drifted, a control signal error is determined by a control circuit part of the feedback loop and is added to the retrieved control signal value. The digital processor measures the value of the control signalerror and derives a new total control signal value that is stored in place of the originally retrieved control signal value.
One limitation of the laser control arrangement described above is that there is no provision for determining the Fabry-Perot resonant frequency to which the laser is being locked without using an external measuring device such as an optical frequency meter.